1. Field of the Invention
The invention relates in general to a protection method for an electronic device and a circuit for detecting positioning signals thereof, and more particularly to a protection method for an electronic device driven by a DC motor and a circuit for detecting positioning signals thereof.
2. Description of the Related Art
DC motors have been widely employed in various electronic products because motor speeds are adjustable easily and can generate a large torque. The electronic devices driven by DC motors such as printers generally utilize optical encoders in company with code strips to generate corresponding signals for the sake of controlling devices.
FIG. 1A shows a portion of a conventional printer. The printer includes a DC motor 110, a control unit (not shown), a print head 120, an optical encoder 130, and a code strip 140. Printing process is performed through the print head 120 driven by the DC motor 110 under the control of the control unit.
When the actuating DC motor 110 drives the print head 120, the optical encoder 130 disposed inside the print head 120 moves along the code strip 140, generating positioning signals A and B. For example, when the optical encoder 130 moves towards the direction pointed to by an arrow 10 under normal situation, the positioning signal A leads the positioning signal B, as shown in FIG. 1B. Conversely, when the optical encoder 130 moves towards the direction pointed to by an arrow 20 under normal situation, the positioning signal B leads the positioning signal A, as shown in FIG. 1C. The control unit controls the DC motor 110 by determining, according to the relationships between the positioning signals A and B, the direction and the distance the print head 120 is moving.
In order to avoid unexpected destructive operations of the DC motor, the control units of some electronic devices with DC motors, conventionally, would include additional protection methods. Currently, most of the protection methods determine whether the devices are in normal state according to variation of a position value which is generated by the control unit after the control unit processes the positioning signals A and B. Since these protection methods only work when the position value does not vary, such protection methods can only avoid abnormal situations due to the failure of generating signals from the optical encoder caused by the dropping of the code strip or the malfunctioning of the optical encoder. When any other situations happen where the positioning signals abnormally generated from the optical encoder, the DC motors would lose control and operates destructively, thus causing unexpected damages to the devices.